fix(read): prevent panic on negative slice index in a key path

GetValue and Exists validated only the upper bound of a numeric key-path
segment, so a negative index such as "arr.-1" passed the check and indexed
the slice with -1, causing an index-out-of-range panic. Add a lower-bound
check (i < 0) to the []int, []string and []any cases in both functions.

testdata/issues59.ini

@@ -1,4 +1,4 @@
-; exported at 2025-08-20 14:11:56
+; exported at 2026-09-06 00:43:11
 
 age = 123
 baseKey = value
@@ -9,8 +9,8 @@ multiWords = hello world
 name = app
 
 [map1]
+key = val
 key1 = val1
 key2 = val2
-key = val